Macrophages are mononuclear phagocytes that play a crucial role in tissue homeostasis and immunity, but also contribute to a broad spectrum of pathologies and thus represent key therapeutic targets. However, in depth insights into the differentiation of primitive macrophages from human pluripotent stem cells, their homeostasis, as well as their defined activities in a tissue-specific context are currently lacking.
According to the mononuclear phagocyte system (MPS) concept, homeostasis of tissue resident macrophages relies on the constant recruitment of blood monocytes. However, while monocytes clearly give rise to macrophages in pathological settings and inflammation, recent evidence has shown that (1) monocytes do not substantially contribute to certain tissue macrophages under steady state and in certain types of inflammation, and (2) some adult tissue macrophages such as brain microglia are derived from embryonic primitive precursors called primitive macrophages that seed tissues prior to birth and maintain themselves in adults by self-renewal.
Based on this evidence, there is a need to recapitulate the embryonic primitive origin of macrophages in vitro in order to design new clinical strategies targeted to monocytes and macrophages. In this regard, the vast majority of existing approaches to hematopoietic differentiation of human cells have utilized either co-culture with a stromal layer, or as embryoid bodies (EBs). Many of these protocols rely on the use of an undefined bovine serum supplement. These conditions introduce intrinsic levels of variability into protocols for hematopoietic differentiation of human cells that can affect reproducibility and yields. Accordingly, there is a need to provide a method for hematopoietic differentiation that overcomes, or at least ameliorates, one or more of the disadvantages described above to recapitulate the embryonic origin of macrophages in vitro.